1. Field of the Invention
This invention relates to a method of prilling. More particularly, it relates to a method of prilling a shear-thinnable mixture of a meltable first component and a second component using mechanical agitation in a prill head to shear thin the mixture.
2. Brief Description of Related Art
Production of mixed fertilizer products such as NPK (nitrogen, phosphorous, potassium) has been typically accomplished by agglomerating basic fertilizer products (potash, urea, etc.) in drum and pan granulation equipment to yield fertilizer particles of desired size and nutrition analysis. Fertilizers for turf and specialty applications are still made in this manner, however, most agricultural customers have moved to simple, blended fertilizers where the basic NPK components are merely mixed together without any attempt to join the particles. While blended fertilizers do not have homogeneous particles with identical NPK content, elimination of the agglomeration step results in reduced manufacturing cost. Uniform spreading of a fertilizer blend, however, is more difficult owing to the differing sizes and weights of the individual basic fertilizer particles used to make the blend.
Mixed, agglomerated fertilizer products can have other benefits besides easier spreading. Micronutrients (trace mineral compounds) can be incorporated into the fertilizer. Because micronutrients are used in minute amounts (less than 1%), it is not practical to blend in these compounds and achieve a uniform spread as the finely ground micronutrient will segregate in the spreading equipment. Another advantage of homogeneous fertilizer is that the properties of the product (such as resistance to humidity in storage) can be controlled by selection of ingredients.
It would be desirable to produce a mixed fertilizer without the added step of a separate agglomeration. The difficulty in accomplishing this goal is that the different basic fertilizers have differing and generally incompatible manufacturing processes. Many fertilizers (e.g.: potash, ammonium sulfate) are crystallized from solution, while others (ammonium nitrate, urea) are prilled from their melt. Crystallized fertilizers either do not melt or have impractical high melting temperatures. Co-crystallization of fertilizers will only work in instances where a compound is formed, otherwise, the least soluble fertilizer will crystallize and the other component will be rejected to the mother liquor as an impurity.
In order to avoid an added agglomeration step, several approaches to produce a mixed fertilizer via prilling have been developed. The advantages of prilling are well known in the art, and include: high percentage of desired product size and thus little recycle, reduced moisture content leading to reduced drying, and excellent sphericity. One approach to producing a mixed fertilizer via prilling is to add a fully soluble component into a fertilizer melt, such as taught in U.S. Pat. No. 3,820,971. This patent restricts the maximum addition of potassium metaphosphate to an ammonium nitrate melt to a soluble quantity in order to permit prilling by standard means. Another approach to producing a mixed fertilizer via prilling is to mix a generally non-melting fertilizer into a fertilizer melt which may be successful if the slurry is made with a low amount of finely ground solids, since the flow characteristics will be nearly identical to a neat fertilizer melt. Therefore, such a melt slurry can be prilled with well-established techniques. However, if a greater amount of non-melting material is desired, then it may become difficult to have the resulting thick mixture flow through conventional prilling systems.
Different approaches have been taken to solve this problem of a thick mixture resulting from a high concentration of a non-melting component in a melt. In some systems of components, there are limited chemical interactions among the components. Double salts, solubility, and freezing point depression are possible results from limited chemical interactions. For instance, U.S. Pat. No. 3,785,796 discloses that the limited solubility of ammonium sulfate in a urea melt resulted in an unexpectedly fluid mixture which could be prilled with a standard rotating bucket prill head, even at ammonium sulfate loadings as high as 70%.
However, in many other systems of components there are chemical interactions that complicate the prilling process. Various methods have been devised to manage these complications by minimizing reaction time. GB 1,481,038 teaches a simple concept of severely limiting the processing time (the period between the time the melt is fed into the mixer to the time the drops are discharged from the prilling apparatus) to 10 seconds or less so that the mixture can be prilled before any detrimental effects develop. U.S. Pat. No. 3,617,235 teaches the use of larger-sized solid particles to slow reaction prior to prilling. U.S. Pat. No. 4,323,386 teaches a method of managing the addition of reagent, delaying full addition of ingredients until just prior to prilling, again to preclude reactions. U.S. Pat. No. 3,856,269 discloses a mixing apparatus to facilitate prilling by providing very rapid but adequate mixing of fertilizer ingredients prior to prilling in a standard perforated spinning bucket.
While effective in the instances cited, limited reaction time methods create disadvantageous restrictions. These methods add cost and complication to the prilling operation by requiring feedstocks with narrow specifications or the expense of engineering some parts or the entirety of the production system to achieve very brief residence times. Another disadvantage of curtailed reaction time is in a situation where a desirable property is created in the reaction of the fertilizer ingredients. The benefit of such a desirable property would be obviated or reduced by curtailing the extent of reaction.
A different approach to prilling high concentrations of a component in a melt slurry is to design prilling equipment that will force the flow of the slurry. Swedish patent 70,119 teaches a vertical screw machine to blend ammonium nitrate melt with ammonium sulfate solid. At the bottom of the screw, the pressure developed by the screw and static head is supplemented by injection behind the spray nozzle. This design is also intended to minimize the reaction time to avoid decomposition. DE 2,355,660 teaches a prill head incorporating a stirrer-impeller mechanical device similar to a centrifugal pump whereby the slurry is introduced at the center and forced under pressure out holes on the circumference of the prilling disk. Disadvantageously, limiting the prill holes to the circumference of the disk reduces production rate; despite being about 600 millimeter (mm) (approximately 2 feet) in diameter, this prill head is reported to produce only 10–12 tons/hour of product. Overall, these mechanical devices involve considerable cost in construction. Furthermore, the abrasive nature of fertilizer slurry will wear the close clearances necessary for efficient pumping, creating a significant maintenance expense.
Mechanical agitation is already in use in mixed fertilizer production. At a minimum, some form of mixing is required to blend the ingredients together and keep the undissolved solids suspended. Previous patents make use or mention the use of agitation. GB 1,481,038 teaches vigorous agitation as an alternative to its use of reaction time control. It states that mixing in excess of six minutes would be required and cites poor mechanical strength and ammonia loss as undesirable outcomes of this method. DE 2,355,660 offers a cylindrical chamber with stirring blades as a comparative. When operated, this configuration resulted in several different problems including: thickening, clogging of the prill holes, nonuniform product, large fraction of reject coarse grains and occasional large agglomerates that did not solidify in the prill tower.
The problem to be solved is to provide a method to prill mixtures which can shear thin without requiring expensive new equipment or additional steps. The invention solves this problem by prilling in which mechanical agitation in the prill head itself reduces the viscosity of the mixture via the mechanism of shear thinning. The thus-thinned mixture can then be prilled in substantially the same manner as pure fertilizers (such as ammonium nitrate and urea). The modified prill head can be readily installed in existing prill towers. Because the agitation is provided in the prill head, no difficulties exist with thickening from chemical reactions, and thus no extensive re-engineering is required to severely limit residence time. Indeed, sufficient residence time can be readily employed in the overall system to achieve desired chemical reactions. Since the prill head agitator need only mix and shear the melt slurry mixture, and not force it through the prill head holes, no close clearances are required in the prill head nor does the structure and motor of the prill head agitator need to be sized to develop pressure in the system. As one example, ammonium sulfate nitrate (ASN) has been successfully prilled using a vertical, agitated prill head and using an agitated rotating bucket prill cup. In practice, 200 rpm (revolutions per minute) rotational speed in a vertical prill head or the simple incorporation of a stationary mixing blade inside a rotating bucket prill cup provides sufficient shear to achieve a prillable viscosity for ASN. The present invention should prove equally useful in any mixture that exhibits shear-thinning behavior.